1. Field of the Invention
The present invention generally relates to two-port isolators, and, more specifically, to a two-port isolator for use in the microwave band and to a communication device.
2. Description of the Related Art
Generally, non-reciprocal circuit elements, such as isolators and circulators, have a characteristic that permits a signal to transmit only in a predetermined direction but not in the opposite direction. Such circuit elements are used in transmitting circuits of mobile communication devices, such as mobile telephones and cellular telephones. One type of non-reciprocal circuit element is the two-port circuit element disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-232818. Equivalent circuits of such a circuit element, namely, a two-port isolator, are shown in FIGS. 11 and 17 of this publication.
The isolator shown in FIG. 17 of this publication, which is well known in the art, has a problem in that two resonance circuits are resonated during signal propagation from an input port to an output port, which causes high power loss and high insertion loss.
In the isolator shown in FIG. 11 of the above-cited publication, on the other hand, resonance circuits disposed between an input port and an output port are not resonated during signal propagation from the input port to the output port, and no power loss occurs, thus greatly reducing the insertion loss.
In the two-port isolator shown in FIG. 11 of the above-cited publication, first and second matching capacitors are formed by laminating high-Q dielectric sheets each having an electrode. This is because the Q factor of the matching capacitors must be high in order to suppress the insertion loss.
However, due to the use of a high-purity starting material and a high-precision manufacturing process, high-Q dielectric material increases the production cost of the isolator. Moreover, high-Q dielectric material generally has a relatively low relative dielectric constant, and it is therefore necessary to increase the area of the matching capacitor electrodes or to increase the number of laminated sheets in order to obtain the required matching capacitors. This makes it difficult to reduce the size and cost of the isolator.
In a case where the matching capacitor electrodes are formed in a multilayer substrate, if the area of via holes connected with the matching capacitor electrodes is small, a large conductor loss occurs in the via holes, and high-Q matching capacitors are not obtained. Thus, via holes must be formed so as to be large enough to provide high-Q matching capacitors. However, since at least a certain clearance is required between a via hole and a matching capacitor electrode formed on a dielectric sheet with the via hole therethrough, the larger the via hole, the smaller the matching capacitor electrode. Thus, the required capacitance is not obtained.